The present invention is related to power and wavelength monitoring for an optical signal, and more specifically to a method and apparatus for monitoring the wavelength and power of an optical laser used in telecommunication applications.
Dense Wavelength Division Multiplexing (DWDM) of optical signals has become a popular method for increasing bandwidth over existing fiber optic backbones. In DWDM systems, multiple signal sources which generate signals of different wavelengths share the same fiber transport system. In effect, the DWDM technology allows a single fiber to function as a plurality of fibers. These signal sources, for example, often share the same optical fiber with spacing between individual channel assignments of only 100 GHz, 50 GHz, or even less, within the 1528 to 1565 nm wavelength range defined by the main gain region of an erbium doped fiber amplifier (EDFA), a typical component of a modern telecommunication system. Assuming this wavelength operating range, the spacing between these source signals is only approximately 0.8 nm, 0.4 nm, or even less.
Semiconductor lasers are commonly used as the signaling sources for telecommunication systems utilizing optical signals. A typical semiconductor laser may be operated in a range of wavelengths depending upon its operating current and temperature. Even at a fixed temperature and current, it is expected that over time, e.g. several years, the wavelength of the light emitted from the laser will gradually shift or drift from the desired operating wavelength to a wavelength that is no longer suitable for the signal""s particular wavelength channel assignment. This shifting or drifting is particularly a problem in systems using DWDM because of the narrow channel assignments for each signal. It should also be noted that the power output of the laser can vary over time, often by as much as a factor of ten over approximately a decade of use. By observing the wavelength shift or drift, a laser""s performance may be corrected by adjusting the temperature and/or current of the semiconductor laser to maintain the semiconductor laser at a desired operating wavelength and power.
Monitoring the wavelength and power of these optical signals, therefore, has become increasingly important as wavelength spacing decreases. Wavelength monitoring can be performed by optical spectrum analyzers that rely on motors to rotate optical gratings or optical filter elements. These devices, however, are often cumbersome when integrated into an optical telecommunication system. Other smaller optical spectrum analyzers suffer from cost restraints. Monitors have been proposed that may be integrated into telecommunication systems, such as U.S. Pat. No. 5,850,292 to Braun for a xe2x80x9cWavelength Monitor For Optical Signals,xe2x80x9d the entirety of which is incorporated herein by reference, but it is still desirable to have an optical wavelength and power monitor that may be integrated into an optical telecommunication system in a cost-effective manner which has a large range over which there is no ambiguity of wavelength and power for an input optical signal.
The present invention is a method and apparatus for determining the wavelength and power of an input optical signal. The wavelength of an input optical signal is determined by dividing the input optical signal into a plurality of individual optical signals and inducing the individual optical signals to interfere with each other to form a plurality of output optical signals. The powers of the output optical signals have a ripple dependence on the wavelength of the input optical signal and the peak amplitude of each of the powers depends monotonically on the wavelength of the input optical signal over a range of wavelengths to be monitored for the input optical signal. The output optical signals are detected, and a plurality of electrical output signals corresponding to the power of each output optical signal, respectively, are produced. The electrical output signals are compared to a predetermined set of signal values, and the wavelength of the input optical signal is determined from the comparison.
By observing a plurality of outputs, the method permits the determination of the wavelength of an input optical signal without relying on the accuracy of a single output. More individual wavelengths may be resolved than different electrical levels at any one output. Further, the wavelength range for which the wavelength of an input optical signal may be determined is expanded because the peak amplitude of each of the powers of the output optical signals depends monotonically on the wavelength of the input optical signal over a range of wavelengths to be monitored for the input optical signal.
An apparatus that may be used to determine the wavelength of an input optical signal according to the present invention includes at least one optical signal divider for dividing the input optical signal into a plurality of individual optical signals, a plurality of optical paths having different optical path lengths disposed such that the individual optical signals propagate through the optical paths, and at least one output coupler. The output coupler accepts a plurality of the individual optical signals after the individual optical signals propagate through the optical paths and permits the individual optical signals to interfere with each other to form a plurality of output optical signals. The powers of the output optical signals have a ripple dependence on the wavelength of the input optical signal. The apparatus also includes at least one slow wavelength dependent coupler and a plurality of optical detectors. The optical detectors are disposed to detect the output optical signals. The slow wavelength dependent coupler causes the peak amplitude of each of the powers to depend monotonically on the wavelength of the input optical signal over a range of wavelengths to be monitored for the input optical signal, and the optical detectors produce a plurality of electrical output signals corresponding to the power of each output optical signal, respectively. The apparatus also includes a means for comparing the electrical output signals to a predetermined set of signal values and a means for determining the wavelength of the input optical signal from the comparison.
The apparatus according to the present invention may be integrated into an optical transmitter system, such as a transmitter system of a telecommunication system utilizing DWDM, in a cost-effective manner while maintaining a large range over which there is no ambiguity of wavelength and power for an input optical signal.
The above and other features of the present invention will be better understood from the following detailed description of the preferred embodiments of the invention which is provided in connection with the accompanying drawings.